The present invention is directed to sonic flow meters including ultrasonic flow meters.
A popular way of making a fluid-flow measurement (i.e., of determining the speed of a fluid or, more typically, its volume rate of flow) in a fluid conduit is to employ an ultrasonic flow meter. Such meters send sound in opposite directions through the same path and measure the transit times required for the sound propagation. If the path has a component in the direction of the fluid flow, the fluid flow causes a difference between the two transit times, and this difference is indicative of the fluid velocity. Integrating the axial component of the fluid velocity over the conduit cross section yields the flow rate.
In principle, one can perform this method by employing transducers that are coupled to the outside surface of the conduit wall, and in fact this approach--that is, of coupling the transducers to the conduit wall rather than using transducers actually in contact with the fluid itself--has significant advantages. Coupling the transducers to the exterior of the conduit avoids the need to de-water the conduit and drill through the conduit wall, as is necessary with a "wetted" transducer, i.e., one that is in direct contact with the fluid. In the case of, for example, conduits used for hydroelectric power plants, the economic loss that results from de-watering can be considerable.
Despite these advantages, it has often been necessary in the past to use wetted transducers instead. The reason for this is that, when an ultrasonic transducer element launches its output through a (usually) high-ultrasound-velocity conduit wall into the relatively low-ultrasound-velocity fluid within the conduit, Snell's law imposes a limit on the angle that the main lobe of the resulting diffraction pattern can form with the normal to the conduit wall. If a measurement path is to be used that significantly exceeds the Snell's-law limit, therefore, the received signal that results from that path tends to be dwarfed by signals received from other paths, and the measurement cannot be made accurately, if at all.
The resultant limitation to small angles would not be a problem if the fluid velocity were strictly axial and the shape of the velocity profile throughout the conduit cross section were known; so long as one knew the speed of sound in the fluid, the conduit cross-sectional area, and the angle that the sound path forms with the conduit axis, the flow-rate computation would be a straightforward matter, and the accuracy would be limited only by the time-measurement resolution.
But the fluid-flow direction is not always strictly axial, and the shape of its profile is rarely predictable. Since the flow direction is not strictly axial, part of the sonic transit-time difference can undesirably result from the non-axial fluid-velocity components, which do not contribute to the flow to be measured. The significance of this "cross-flow" error is greater when the non-axial component of the ultrasound path is large in comparison with the axial component, as it is at the path angles that are possible with prior-art externally mounted flow meters. Still, the cross-flow problem can be overcome by employing crossed sound-propagation paths.
The more-difficult problem results from the unpredictable nature of the velocity profile. To obtain high accuracy, one must measure the transit-time difference not for a single diametral path but rather for a plurality of chordal paths so as to "sample" the average velocities at various slices through the conduit cross section. This means that ultrasound would have to be introduced at an angle to the wall-surface normal even if no axial ultrasonic-path component were necessary. As was mentioned above, Snell's law places a limit on how large this angle can be in traditional externally mounted flow meters. For accurate measurements, therefore, it has been necessary in the past to resort to wetted-transducer flow meters.